Method of utilizing the heat content of a combustible gas during refining pig iron



y 1967 K. RIEDER 3,321,302

METHOD OF UTILIZING THE HEAT CONTENT OF A COMBUSTIBLE GAS DURING REFINING PIG IRON Filed Dec. 18, 1964 6 Sheets-Sheet 2 8 9 blowing il'rne 1h III/hula:

INVENTOR KARL RIEDER HIS ATTORNEYS May 23, 1967 K. RIEDER 3,321,302

METHOD OF UTILIZING THE HEAT CONTENT 0F A COMBUSTIBLE AS DURING REFINING PIG IRON Filed Dec. 18, 1964 6 Sheets-Sheet 3 "a K Q 5 Q f 5 (Q I r l/ i I I I! 5/ I0 4 Q (9 I 5 Q 5 I m ll- V w O D m v O) s g m/ A Q34: In l\ A m INVENTOR KARL RIEDER M.%.%ZIPAM HIS ATTORNEYS May 23, 1967 K. RIEDER 3,321,302

METHOD OF UTILIZING THE HEAT CONTENT OF A COMBUSTIBLE GAS DURING REFINING PIG IRON Filed Dec. I8, 1964 6 Sheets-Sheet 4 00s 05',- 009 as; 00: 052 0'0; 05/

INVENTOR KARL RIEDER BY HIS ATTORNEYS May 23, 1967 K. RlEDER 3,321,302

METHOD OF UTILIZING THE HEAT CONTENT OF A COMBUSTIBLE GAS DURING REFINING PIG IRON HIS ATTORNEYS May 23, 1967 K. RIEDER 3,321,302

METHOD OF UTILIZING THE HEAT CONTENT OF A COMBUSTIBLE GAS DURING REFINING PIG IRON Filed Dec. 18, 1964 e Sheets-Sheet 6 ,vuaaaad Ujul/ w m pJopunls INVENTOR KARL RIEDER H IS ATTORNEYS United States Patent 3,321,302 METHOD OF UTILIZING THE HEAT CONTENT OF A COMBUSTHBLE GAS DURING REFINING PEG IRON Karl Rieder, Linz, Austria, assignor to Vereinigte Osterreichisclie Eisenund Stahlwerke Aktiengesellschaft, Linz, Austria, a company of Austria Filed Dec. 18, 1964, Ser. No. 419,449 Claims priority, application Austria, Dec. 23, 1963, A 10,37 6/ 63 3 Claims. (Cl. 75-60) This invention relates to a method of utilizing the heat content of a combustible gas during refining pig iron with pure oxygen, the oxygen being blown onto the pig iron bath from above.

Various proposals have already been made to utilize the sensible heat of converter waste gases, on the one hand, and the heat of combustion of the carbon oxide contained in the waste gases, on the other hand. To this end, it has previously been proposed by the applicant to add oxygen in the form of infiltrated air to the waste gases escaping from the converter mouth, the air being stoichiometrically proportioned to provide for a complete combustion of the carbon oxide. The mixture of waste gases and combustion gases was passed through a chimney including a boiler system and then through a dust removing plant and exhausted into the open.

According to a very old proposal dating back to 1909 (German patent specification No. 216,302) it has been known, on the other hand, to recover a combustible gas from the waste gases from a converter by preventing the access of air and thereby a combustion of the carbon oxide contained in the converter gases by a water-cooled hood placed on the converter mouth. The converter gases are trapped only during the carbon combustion period so that the waste gases poor in CO evolved during the desiliconisation period and during the dephosphorisation period will not reduce the calorific value of the gas to be trapped.

Based in the mentioned principle several further proposals have become known in recent times. According to Austrian Patent No. 205,527, for instance, a combustible gas is obtained during the carbon combustion period. Cold nitrogen or an other cold, non-oxidizing gas is blown into a cooled hood arranged on the converter mouth, in order to reduce the temperature of the converter waste gases and to avoid damage to the pipe-lines. A modification of the said proposal is disclosed in the patent oi addition No. 229,347, which relates to substantially the same method, with the qualification that the waste gases are trapped and stored only when they have a certain minimum content of carbon oxide of, e.g., 25%. The first and last runnings are exhausted into the open air.

Further proposals which have become known from French YAWATA patents and from Fume Arrestment. published by Iron & Steel Institute, 1963, relate to the recovery of combustible gases from the converter waste gases evolved in top-blowing processes, the method comprising flushing with nitrogen before the collection period and introducing nitrogen into the exhausting system also after the collection period, to the control of the pressure in the exhausting system and to the adjustment of a certain pressure difference between inside pressure and outside pressure, to scaling the gap between the converter mouth and the hood, and to other details.

As is evident from the above presentation of the prior art, two entirely different ways of utilizing the energy content of the waste gases have been attempted. One way resides in combining the converter with a waste heat boiler, burning the carbon oxide contained in the converter waste gases and utilizing the total of the sensible heat and of the heat of combustion of the waste gases. The second way is to obtain waste gases without combustion, where the converter does not cooperate with a waste heat boiler, the waste gases merely being cooled, the first and last runnings discarded, and only the waste gases evolved during the combustion of carbon and having a high CO content being collected after having passed washers, purifyers etc.

Both of the ways hitherto attempted involve certain disadvantages. The disadvantage of the first method is seen in the fact that the escaping waste gases having a temperature of 1600 to 1800 C. are increased in temperature due to the combustion heat, for which reason the installations in the lower part of the chimney have to be made of a highly refractory and wear-resistant material. The amount of heat accumulating in the waste heat boiler and, accordingly, the steam output are increased and decreased by bounds in the same rate as carbon oxide is produced, and it is a known fact that, e.g., between the 6th and 12th minutes the amount of heat obtained is doubled and from the 13th to the 18th minute is reduced by half. On the other hand, the advantage of the first method is obvious, namely, a high operational safety, explosions being eliminated.

The disadvantage of the second method may be seen in that the sensitive (physical) heat of the hot waste gases is not or may not sufiiciently be utilized, that complicated control means are necessary to exclude the air, that also in periods in which there is no trapping, a certain minimum amount of an inert gas has to be circulated through the exhausting system in order to keep the control means in operation.

It is an object of the present invention to avoid the disadvantages of the known systems and combine their advantages. This is accomplished according to the invention in that, in a process for utilizing the heat content of a combustible gas during a process of refining pig iron by means of oxygen wherein the oxygen is blown from.

above onto the pig iron bath and the waste gases evolved during the combustion of carbon, after having escaped from the converter, are passed through a chimney including a waste heat boiler system and subsequently stored, an oxygen-containing gas is added to the waste gases before they enter into the chimney, the oxygen content of said gas being controlled during the blowing period to vary between and 7% by volume-cg. by pro-mixing nitrogen and oxygen-in such manner that the oxygen content is continually decreased during the first part of the blowing period and continually increased during the second part of the blowing period to enable a partial combustion of the carbon monoxide contained in the waste gases in the chimney so that the total of sensible heat of the waste gases produced and the heat of combustion generated during the major part of the blowing period is constant, and consequently also the steam production remains constant during that period. Suitably, the oxygen content of the gas to be added to the waste gases is controlled in dependence on the progress of the refining reaction: From the 2nd to the 11th minute the oxygen content may be gradually decreased from 100% to 7%, and from the 16th to the 20th minute it may be increased again from 7% to 100%. The exact content and time is, of course, dependent on the size or capacity of the crucible or converter. The mentioned data apply exactly to a 50 ton crucible. With a reduction or extension of the blowing periods they are varied accordingly.

It is also within the scope of the method of the invention to vary the amount of gas added. In the first phase of the refining process, for instance from the 2nd to the 10th minute, the amount may be increased, and in the subsequent phase, i.e. from the 10th to the 20th minute, it may be continuously decreased again.

The formation of the gas having a varying content of oxygen is preferably effected by pre-mixing oxygen and nitrogen. Thus, an amount of nitrogen, which is increased minute by minute until the middle of the refining process (12th minute), e.g. from to 100 standard cubic metres per minute, may be added to an amount of oxygen which is kept approximately constant at, e.g., 8 to 10 standard cubic metres per minute, and from this maximum a decreasing amount of nitrogen, decreasing from 100 standard cubic metres to 0 standard cubic metres per minute, may be added.

The method according to the invention has the advantage that the sensible heat of the converter waste gases produced is fully utilized in the waste heat boiler and that, moreover, an adjustable amount of heat of combustion is utilized, sudden loads on the waste heat boiler and excessive heating being avoided. Depending on the degrees of utilization of additional combustion energy desired in the waste heat boiler and on the desired composition of the Waste gases trapped behind the boiler, respectively, the calorific value of the gas collected may be adjusted, e.g., to a value of 1100 kcal./standard cubic metre, 1700 kcal./standard cubic metre or 2000 kcaL/ standard cubic metre.

The risk of explosion is substantially avoided by the method of the invention, because at any time in which the gas is in the chimney, the ignition temperature is exceeded and the carbon oxide has fully reacted with the oxygen in the addition gas. Behind the chimney the gas to be trapped is in any case free of oxygen so that explosive mixtures can be no longer be formed. Also the control in the exhaustion system is much simpler behind the chimney than in the known apparatus in which the valves had to be arranged in and behind the water-cooled hood. This advantage is due to the fact that the gas to be trapped behind the chimney already has a very low temperature of approximately 200 to 300 C., so that the valves and the control system are not exposed to such high stresses as when they have to operate at temperatures ranging at 800 or 1000 C.

The method according to the invention is explained in more detail in connection with the accompanying drawings, in which FIG. 1 is a graph illustrating the amount of flue gases produced during a top blowing process in dependence on time, its calorific value which is constantly kept at 2000 kcaL/standard cubic metre during a definite period of the refining process, the amount and composition of the addition gas in dependence on time, and the amount of heat supplied by the flue gas to the boiler system in the chimney. FIG. 2 is a graph illustrating the control of the amount and composition of the addition gas added to the converter waste gases, in dependence on time.

FIGS. 3 and 4 and FIGS. 5 and 6 are graphs applying to the production of flue gases having a calorific value constantly maintained at 1700 kcal./ standard cubic metre and 1100 kcaL/standard cubic metre, respectively, during a definite period of the refining process.

In particular, the following characteristics of the method are apparent from the accompanying graphs:

When carrying out a top-blowing process in a 50 ton crucible, using a charge consisting of steel-making ig iron, the refining period is 21 minutes. The blowing time in minutes is plotted on the abscissa. The ordinate of the graph contains three scales, viz: the amount of heat sup plied to the boiler system in the chimney in thermal units (kcal.) per minute, the amounts of gases added and recovered, respectively, per unit of time in standard cubic metres per minute, and the calorific value of the fiue gas and of the collected gas in thermal units (kcaL) per standard cubic metre.

Curve 1 represents the calorific value which the flue gas produced during refining under the mentioned conditions would have, if no additional measures were taken. As is evident, the curve rises considerably until the 12th minute and then strongly declines, which means that, if this gas were trapped, the calorific value thereof would vary considerably; similarly the heat supplied to the boiler system would correspond to the rise and fall, respectively, of the curve.

The embodiment of the method according to the invention as illustrated in FIG. 1 is based on the assumption that from the 1st to the 19th minute a constant amount of heat of 145,000 kcal./min. is supplied to the boiler. This area corresponding to the amount of heat delivered is bordered by curve 2. The addition according to the invention of an oxygen-containing addition gas is shown by curves 3 and 4, curve 3 illustrating the amount of oxygen in standard cubic metres per minute and curve 4 illustrating the amount of nitrogen added in standard cubic metres per minute. The amount of flue gas obtained under these conditions is indicated by curve 5. Curve 6 defines an area which corresponds to the calorific value of the flue gas collected. As is shown, the calorific value of the flue gas is continuously increased until the 3rd minute and reaches the desired value of 2000 kcal./ standard cubic metre between the 3rd and 4th minute. This value is maintained constant until the 17th minute.

The varying composition of the addition gas from oxygen and nitrogen is controlled, according to the invention, as is indicated by curves 3 and 4. This control is illustrated more clearly by FIG. 2. In FIG. 2 numeral 3 again denotes the amount of the oxygen added, and 7 designates the total of oxygen and nitrogen. Curve 3 of FIG. 2 exactly corresponds in behaviour to the trend of curve 3 in FIG. 1, and curve 7 represents the sum of curves 3 and 4 of FIG. 1. Accordingly, the behaviour of curve 7 in those phases in which pure oxygen is added, i.e. from the 1st to the 3rd minute and from the 18th to the 21st minute, is exactly the same as that of the oxygen graph in FIG. 1. Curve 8 indicates the percentage of oxygen in the addition gas.

Thus the following course of the process may be read from the graphs. The amount of oxygen (curve 3) is as follows:

at the end of the 1st minute, 15 standard cubic metres/ minute at the end of the 2nd minute, 12 standard cubic metres/ minute at the end of the 3rd minute, 10 standard cubic metres/ minute at the end of the 4th minute, 10 standard cubic metres/ minute at the end of the 12th minute, 10 standard cubic metres/ minute at the end of the 19th minute, 20 standard cubic me-tres/ minute and the corresponding amounts of nitrogen, according to graph 4, are:

at the end of the 1st minute, 0 standard cubic metre/ minute at the end of the 2nd minute, 0 standard cubic metre/ minute at the end of the 3rd minute, 0 standard cubic metre/ minute at the end of the 4th minute, 5 standard cubic metres/ minute at the end of the 12th minute, standard subic metres) minute at the end of the 19th, 20th and 21st minute, 0 standard cubic metre/ minute Accordingly the total addition gas (FIG. 2) is, for instance, at the end of the 12th minute standard cubic metre/minute. The composition of the addition gas (curve 8) exhibits, until the end of the 3rd minute, an oxygen content of 100%, at the end of the 4th minute 50%, at the end of the 12th minute 9%, at the end of the 16th minute 7%; then the oxygen content increases again and at the end of the 17th minute amounts to 14%, and at the end of the 18th, 19th, 20th and 21st minute to 100%.

In FIGS. 3 and 4 and 5 and 6 corresponding curves are shown for the production of flue gases having a calorific value of 1700 kcaL/standard cubic metre, and 1100 kcaL/standard cubic metre, respectively assuming again that the flue gases, before being trapped, have supplied from the 2nd to the 19th minute a constant amount of heat of 145,000 kcal./min. to the boiler.

What I claim is:

1. In a method for utilizing the heat content of a combustible gas during refining crude iron by means of oxygen blown from above onto the crude iron bath and evolving waste gases during the combustion of carbon, said waste gases escaping from the converter and passing through a chimney including a waste heat boiler system and subsequently being stored, the steps comprising adding an oxygen-containing gas to the waste gases before they enter the chimney, the oxygen content of said gases being controlled during the blowing period to vary between about 100% and about 7% by volume in such manner that the oxygen content is continually decreased during the first part of the blowing period and continually increased during the second part of the blowing period to enable a partial combustion of the carbon monoxide contained in the waste gases in the chimney so that the total of sensible heat of the waste gases produced and the heat of combustion generated during the major part of the blowing period is maintained substantially constant.

2. A method according to claim 1 in which the refining process is carried out in a ton converter and the oxygen content of the added gas is gradually decreased from the 2nd to the 16th minute from about 100% to about 7% and then increased from the 16th to the 20th minute from about 7% to about 100%.

3. A method according to claim 1 in which the amount of oxygen is maintained substantially constant from the 2nd to the 20th minute at about 10 to 15 standard cubic meters per minute and nitrogen ,is mixed with said oxygen in varying amounts to obtain a constant calorific value of the gas collected.

References Cited by the Examiner UNITED STATES PATENTS 2,831,467 4/1958 Guczky 60 3,222,045 12/1965 Spetzler 75-60 FOREIGN PATENTS 1,005,590 9/1965 Great Britain.

BENJAMIN HENKIN, Primary Examiner. DAVID L. RECK, Examiner. 

1. A METHOD FOR UTILIZING THE HEAT CONTENT OF A COMBUSTIBLE GAS DURING REFINING CRUDE IRON BY MEANS OF OXYGEN BLOWN FROM ABOVE INTO THE CRUDE IRON BATH AND EVOLVING WASTE GASES DURING THE COMBUSTION OF CARBON, SAID WASTE GASES ESCAPING FROM THE CONVERTER AND PASSING THROUGH A CHIMNEY INCLUDING A WASTE HEAT BOILER SYSTEM AND SUBSEQUENTLY BEING STORED, THE STEPS COMPRISING ADDING AN OXYGEN-CONTAINING GAS TO THE WASTE GASES BEFORE THEY ENTER THE CHIMNEY, THE OXYGEN CONTENT OF SAID GASES BEING CONTROLLED DURING THE BLOWING PERIOD TO VARY BETWEEN ABOUT 100% AND ABOUT 7% BY VOLUME IN SUCH MANNER THAT THE OXYGEN CONTGENT IS CONTNINUALLY DECREASED DURING THE FIRST PART OF THE BLOWING PERIOD AND CONTINUALLY INCREASED DURINF THE SECOND PART OF THE BLOWING PERIOD TO ENABLE A PARTIAL COMBUSTION OF THE CARBON MONOXIDE CONTAINED IN THE WASTE GASES IN THE CHIMNEY SO THAT THE TOTAL OF SENSISBLE HEAT OF THE WATER GASES PRODUCED AND THE HEAT OF COMBUSTION GENERATED DURING THE MAJOR PART OF THE BLOWING PERIOD IS MAINTAINED SUBSTANTIALLY CONSTANT. 